1. Field of the Invention
The present invention relates to a method to determine the pore size distribution on the surface of porous materials. In particular, this method measures the size of the pore-mouth rather than that of the pore-throat (i.e., the narrowest position) in the pore tunnels.
2. Related Art
Because of their internal porosity, porous materials have been widely applied in industries (such as petrochemistry, food, construction, metallurgy, aerospace, etc.) for separation and purification, gas distribution, catalysts, noise silencing, shock absorption, shielding, heat exchange, electrochemistry, and so on. One important use of porous materials is as porous membranes with symmetric or asymmetric structure. In recent years, attention has been paid to the concept of composite membranes, which consist of a functional membrane layer on a substrate material. For filtration, the membrane pore size is very significant. Here the term “pore size” mainly refers to the narrowest position in a pore tunnel, i.e., the “pore-throat”, as shown in FIG. 1. In contrast, the “pore-mouth” means the mouth of a pore on a surface. This is more important for coatings on the surface of the porous material (in most of the cases, the pore-throat is significantly smaller than the pore-mouth), and a typical example is the fabrication of palladium composite membranes.
Palladium membranes (including palladium alloy membranes) are highly permeable to hydrogen and have been applied to hydrogen purification for several decades. Except for hydrogen and its isotopes, no other gases can pass through the membranes. The commercialized membranes are self-stand ones with a thickness of around 100 μm or larger. Although it is highly desired to reduce the membrane thickness, to which the membrane permeability is inversely proportional, reduction is limited by the poor physical strength of the membrane and the manufacturing cost. An ideal solution is the introduction of a composite membrane concept, i.e., deposition of a thin layer of palladium or palladium alloy on a porous substrate (e.g., porous ceramics and porous stainless steel), whereby the membrane thickness can be decreased to a couple of microns. Accordingly, the membrane permeability can be increased by one order of magnitude, and the consumption of the noble metal is also decreased. However, a challenging problem is the appearance of membrane defects. The larger the pore-mouth of the substrate material, the thicker the membrane has to be. To fabricate a palladium membrane that is completely pinhole-free, particular attention must be paid to the largest pore and, more specifically, the largest pore-mouth of the substrate [Yu J, Hu X, Huang Y. Ceramic modifications of the porous stainless-steel surface toward the Palladium membranes for hydrogen separation. Prog. Chem., 2008, 20(7/8): 1208-1215.].
There are many techniques for the determination of the pore size distribution of a porous material [Hernández A, Calvo J, Prádanos P, Tejerina F. Pore size distributions of track-etched membranes; comparison of surface and bulk porosities. Colloids Surf. A, 1998, 138: 391-401.] [Zhang Q, Zhang Z, Wei H. Characterization methods of porous material filter rating. Filter & Separator, 2000, 10: 33-37.], such as mercury intrusion porosimetry, the bubble point method, liquid-liquid displacement, suspension filtration, and gas permeation. Mercury intrusion porosimetry probes the pore size distribution via monitoring the volume of mercury that the sample absorbs at different pressures. This method, however, also detects the blind pores, which are not significant for filtration. Therefore, the results may not be consistent with the real filtration effects. Moreover, mercury intrusion porosimetry is not suitable for a membrane with asymmetric structure.
The principle of the bubble point method is as follows [ASTM F316-2003 Standard test methods for pore size characteristics of membrane filters by bubble point and mean flow pore test.] [ISO 4003-1990 Permeable sintered metal materials determination of bubble test pore size.] [GB/T 1967-1996 Test method for pore diameter of porous ceramics.] [GB 5249-1985 Permeable sintered metal materials; Determination of bubble test pore size.] [Huang P, Xing W, Xu N, Shi J. Pore size distribution determination of inorganic microfiltration membrane by the gas bubble pressure method. Technology of Water Treatment, 1996, 22: 80-84.]. When a pore tunnel is blocked with a wetting agent, a certain pressure is necessary for a gas to reopen the pore tunnel because of the surface tension of the wetting agent, and the smaller the pore size, the larger the gas pressure that will be required. Therefore, an increase in the gas pressure will successively reopen the pore tunnels with decreasing pore size. The first pore to be reopened is the largest, i.e., the “bubble point”. This method works by measuring the gas flowrate versus pressure at the dry and wetted states of the sample, and the pore size distribution can then be calculated according to a theoretical model. Noticeably, the pore size given by the bubble point method actually refers to the “pore-throat”. In most of the cases, the shape of the pore tunnel is irregular, and the so-called “pore size” actually refers to the diameter of a circle whose area is the same as the cross-sectional area of the pore-throat.
The principle of the liquid-liquid displacement is the same as that of the bubble point method, except that the gas is replaced with a liquid that is insoluble in the wetting agent. This method also probes the pore-throat size.
To measure the pore size through suspension filtration, a filtration of a suspension containing spherical solid particles is performed under laminar flow, and the size distribution of the particles remaining in the suspension is analyzed before and after filtration. The largest pore size of the filter is determined by the diameter of the largest particle that passes through the filter. The definition of the pore size given by this method refers to the diameter of an inscribed circle at the pore-throat. For a non-circular pore-throat, its size measured through suspension filtration will be smaller than that given by the bubble point method.
Except for mercury intrusion porosimetry, all of the above methods provide the pore-throat size distribution, which is important for filtration performance study. However, none of them probe the pore-mouth size distribution, and, so far, there have been no good solutions. Direct observation by microscopy (for example, SEM) can be considered in laboratory studies, but it is limited by the small visual field and generally only applicable for small specimens. Moreover, the direct microscopic observations cannot locate the largest pore-mouth of the whole sample quickly.